Nitrosyl chloride is a compound commonly used in chlorination, diazotization and nitrosation reactions of organic compounds.
In particular, it is employed in the industrial production of the oxime of cyclododecanone by photonitrosation of cyclododecane, which oxime is an intermediate in the synthesis of lauryllactam, which constitutes the base monomer for polyamide 12.
Various methods for the preparation of nitrosyl chloride are already known.
Mention may be made, among those which are suitable for industrial production, of:
the process which consists in reacting nitric acid, nitrogen monoxide and hydrochloric acid or a metal chloride (see, for example, U.S. Pat. No. 3,214,240), PA1 the process which consists in reacting aqueous hydrochloric acid with gaseous nitrogen dioxide (see, for example, BE-B-626,426), and PA1 the process, in two stages, which consists in reacting a gaseous mixture of nitrogen monoxide and of nitrogen dioxide with sulphuric acid, so as to obtain a solution of nitrosylsulphuric acid in a mixture of sulphuric acid and water, and in then reacting this solution with gaseous hydrogen chloride at a temperature of 50 to 250.degree. C. under conditions such that the water content of the solution is between 2 and 13% (see FR 1,343,113).
When the last process is employed, there may be an inducement to carry out the abovementioned stages on geographically separate industrial sites. It is then necessary to carry out operations of transportation and of storage of the nitrosylsulphuric acid solution while waiting for it to be employed in the following stage.
During these operations, disadvantages become apparent.
Solutions containing more than 40% of nitrosylsulphuric acid crystallize when the water content exceeds 10%. To overcome this disadvantage, it is then necessary either to reduce the content of nitrosylsulphuric acid, which is reflected by a decrease in the productivity, or to heat the solution to a temperature of the order of 50 to 100.degree. C., which is expensive.
The presence of water causes, in the more or less long term, hydrolysis of the nitrosylsulphuric acid, which correspondingly decreases the yield of nitrosyl chloride. Hydrolysis becomes more significant as the temperature rises.
It has now been found that it is possible to overcome the abovementioned disadvantages, without significantly affecting the conversion to nitrosyl chloride, by reacting a substantially water-free solution of nitrosylsulphuric acid in sulphuric acid with gaseous hydrogen chloride and aqueous hydrochloric acid.
The subject-matter of the present invention is a process for the preparation of nitrosyl chloride from a solution of nitrosylsulphuric acid in sulphuric acid and from hydrogen chloride, the said process being characterized in that the solution of nitrosylsulphuric acid is substantially free of water and that aqueous hydrochloric acid and gaseous hydrogen chloride are used in amounts such that the molar ratio of aqueous hydrochloric acid to nitrosylsulphuric acid is between 0.25 and 1.2 and the molar ratio of aqueous hydrochloric acid and of gaseous hydrogen chloride to nitrosylsulphuric acid is between 1.25 and 5.
More particularly, the invention relates to such a process in which the molar ratio of aqueous hydrochloric acid to nitrosylsulphuric acid is between 0.27 and 1.1 and the molar ratio of aqueous hydrochloric acid and of gaseous hydrogen chloride to nitrosylsulphuric acid is between 1.4 and 5.
Better still, in the process according to the invention, the molar ratio of aqueous hydrochloric acid to nitrosylsulphuric acid is between 0.35 and 0.50 and the molar ratio of aqueous hydrochloric acid and of gaseous hydrogen chloride to nitrosylsulphuric acid is between 1.5 and 2.5.
The process will be better understood in the light of the description which follows.
The solution of nitrosylsulphuric acid in sulphuric acid generally comprises 40 to 73% by weight of nitrosylsulphuric acid and preferably 55% to 70% by weight. It can optionally comprise up to 3% by weight of sulphur trioxide and up to 2% by weight of water.
The solution of nitrosylsulphuric acid in sulphuric acid can be obtained, according to methods well known by the man skilled in the art, for example, by reaction of nitric acid and of sulphur dioxide, or by reaction of nitrogen monoxide, of nitrogen dioxide and of sulphuric acid (see, for example, FR 1,343,113), and preferably by reaction of nitrogen monoxide, of nitrogen dioxide and of oleum. It is also possible to replace the mixture of nitrogen monoxide and of nitrogen dioxide with nitrogen sesquioxide (N.sub.2 O.sub.3).
The aqueous hydrogen chloride employed in the process according to the invention comprises 20 to 37% by weight of hydrogen chloride and preferably 30 to 37% by weight.
It is advantageous to add a, preferably inert, stripping gas to the gaseous hydrogen chloride, the aim of which is to improve the extraction of the nitrosyl chloride from the reaction mixture. Mention may be made, as example of such a gas, of nitrogen, argon, methane and mixtures of these gases.
The reaction temperature is generally between 10 and 200.degree. C. and preferably 20 and 100.degree. C.
The process according to the invention can be employed in any type of device known to a person skilled in the art, such as a reactor or a column for liquid/gas exchange, for example containing plates. Use is advantageously made of a plate column.
The process according to the invention can operate batchwise or continuously. Continuous operation is preferred.
The conversion of nitrosylsulphuric acid to nitrosyl chloride is at least equal to 97%, preferably 99% and better still 99.5%.
The nitrosyl chloride recovered in the gaseous mixture comprises a low nitrogen oxide content (less than 1000 ppm by weight).
The nitrosyl chloride can be used as is, that is to say as a mixture with the gaseous hydrogen chloride and, if appropriate, the stripping gas, or can be subjected to a purification stage, for example by cooling, so as to obtain the nitrosyl chloride in the liquid state, and low-temperature distillation.
The liquid effluent essentially comprises sulphuric acid in solution in water. The concentration of the acid, which varies according to the amount of aqueous hydrogen chloride employed, is generally between 60 and 90% by weight.